Make scrollbars only visible when a Div is hovered over?

Question

I am trying to figure out how to have a scrollable div that only shows its scrollbars when Hovered.

Example is Google Image search, in the image below you can see

Answer 1

This will work:

#div{
     max-height:300px;
     overflow:hidden;
}
#div:hover{
     overflow-y:scroll;
}